img.pixel-art {
image-rendering: pixelated;
}Scale the image with whole multiples (200%, 300%) and every pixel stays a sharp square. Fractional scales make some pixels bigger than others, which looks uneven.
If you need a fallback for old Firefox, image-rendering: crisp-edges does the same there. On this site every PNG and GIF in a post gets the pixelated treatment; photos are JPEGs, so they stay smooth.